Unique Number of Occurrences — C++

// Time:  O(n)
// Space: O(n)

class Solution {
public:
    bool uniqueOccurrences(vector<int>& arr) {
        unordered_map<int, int> count;
        for (const auto& a : arr) {
            ++count[a];
        }
        unordered_set<int> lookup;
        for (const auto& [k, v] : count) {
            if (lookup.count(v)) {
                return false;
            }
            lookup.emplace(v);
        }
        return true;
    }
};